Chapter 3 Summary

In this chapter of Your Inner Fish, the author delves deeply into why organisms develop the way they do and what separates them form any other living organism.  The answer to the second part of that is within the DNA; organisms may have extremely similar DNA but the relatively small variations make huge differences. This is because of development also, when an organism is first created it is very small and has DNA that can only form so many cells.  Those cells though, then form more and more and eventually what started as a single cell organism is a self-functioning creature.  So this is why creatures with similar structures differ so greatly in the big picture.  The next question that is answered is why creatures have symmetrical structures.   A breakthrough in research at a Harvard research lab explained why pinky's formed different from thumbs and why hands formed symmetrically.  The zone of polarizing activity (ZPA), was where the development was happening that would make a the hands.  If moved, a may form two identical digits.  Another project where the ZPA was secluded to half of a cell showed that the half with the ZPA developed a hand while the other side did not.  In the final part of the reading it explained that because of the way certain parts of genes in DNA are active at only certain times to make different types of molecules, even humans and fly's have similar genes for hands/wings, but they develop differently.  It argues that the changes through evolution did not necessarily come from new DNA, but rather ancient DNA that was not active in other organisms during their developmental stages.